Method and apparatus for precise temperature control

ABSTRACT

A temperature control system for maintaining a cryogenic load at a predetermined temperature by circulating a wet mixture of a coolant through tubing that is in heat-transfer relationship with the load while maintaining the coolant in a saturated condition throughout the tubing at a constant temperature that is slightly cooler than the predetermined load temperature. The system includes a preconditioning chamber from which liquid coolant is supplied to the tubing. The chamber is partially filled with liquid coolant, with the remainder of the chamber filled with vaporous coolant. Inlet and outlet metering valves for the tubing are mounted in the chamber and controls are provided for maintaining the entire interior environment of the chamber including the valves, interconnecting tubing and both the liquid and vaporous coolant in a state of thermodynamic equilibrium at a temperature that is a few tenths of a degree lower than the predetermined temperature of the load. Sensors are provided at the load and in the tubing for actuating controllers that manipulate the valves to regulate the coolant flow to be at a rate that maintains the coolant in its saturated wet-mixture condition in all sections of the tubing during heat transfer from the load to the coolant.

[451 June 6, 1972 METHOD AND APPARATUS FOR PRECISE TEMPERATURE CONTROLSteven J. St. Lorant, San Mateo, Calif.; Phillip L. Dow, Geneva,Switzerland The United States of America as represented by the UnitedStates Atomic Energy Commission Filed: Dec. 4, 1970 Appl. No.: 95,237

Inventors:

Assignee:

US. Cl ..62/208, 62/467, 62/514, 62/56, 62/209, 62/216, 165/2 Int. Cl..F25b 41/00 Field of Search ..62/514, 514 A, 467, 45, 56, 62/216, 208,209; 165/2 References Cited UNITED STATES PATENTS 8/1961 Goodenough..62/514 7/1965 London 7/ 1965 Steinhardt 9/1966 Bourquard.... 2/1969HEATER POWER SOURCE CONTROL COOLANT SUPPLY Primary Examiner-William J.Wye AttorneyRoland A. Anderson ABSTRACT A temperature control system formaintaining a cryogenic load at a predetermined temperature bycirculating a wet mixture of a coolant through tubing that is inheat-transfer relationship with the load while maintaining the coolantin a saturated condition throughout the tubing at a constant temperaturethat is slightly cooler than the predetermined load temperature. Thesystem includes a preconditioning chamber from which liquid coolant issupplied to the tubing. The chamber is partially filled with liquidcoolant. with the remainder of the chamber filled with vaporous coolant.lnlet and outlet metering valves for the tubing are mounted in thechamber and controls are provided for maintaining the entire interiorenvironment of the chamber including the valves, interconnecting tubingand both the liquid and vaporous coolant in a state of thermodynamicequilibrium at a temperature that is a few tenths of a degree lower thanthe predetermined temperature of the load. Sensors are provided at theload and in the tubing for actuating controllers that manipulate thevalves to regulate the coolant flow to be at a rate that maintains thecoolant in its saturated wet-mixture condition in all sections of thetubing during heat transfer from the load to the coolant.

7 Claims, 1 Drawing Figure METHOD AND APPARATUS FOR PRECISE TEMPERATURECONTROL BACKGROUND OF THE INVENTION The present invention relates to asystem for maintaining a load at a set precise temperature by means of acoolant that is in a saturated wet-mixture condition at all points ofheattransfer relationship with the load, and more particularly, itrelates to a temperature control system in which the coolant in both itsliquid and vaporous states as well as inlet and outlet metering valvesand interconnecting tubing to the load are all maintained in a state ofthermodynamic equilibrium.

For successful operation of certain types of heat loads, it is necessaryto maintain the load homogeneously constant within very close toleranceseven in the presence of sudden load variations. For example, in a liquidhydrogen bubble chamber that is used in conjunction with a high-energyparticle accelerator, the paths of subatomic particles are made visiblein the form of a series of small bubbles in the hydrogen. Pictures forlater study are rapidly taken of the paths before the bubbles disperse.However, an inhomogeneous change in the range of l/ F of the liquidhydrogen can cause variations in the density of the mass of hydrogen.The density variations cause convection of the hydrogen; and sincemasses of differing density refract light at different angles, the pathsof the particles in the moving hydrogen masses appear to be unevenlydisplaced and distorted and any reproductions made of the paths will beinaccurate. It is desirable therefore to precisely maintain thetemperature of a bubble chamber uniform and constant. For example, it isfound that to obtain satisfactory pictures, the chamber should bemaintained within l/lOO" F. Preferably, this is accomplished with aminimum of cryogen and without resort to complex and expensivearrangements for compensation of the relatively large temperaturedifferentials that normally developin the pumps, valves, heat exchangersand interconnecting lines, which differentials, if not compensated,result in thermal leaks to the load.

SUMMARY OF THE INVENTION In brief, the present invention pertains to atemperature control system for precisely maintaining a heat load at apredetermined temperature and in general, the system includes aheat-emitting mass arranged as the heat load within the system, a supplyof liquid coolant, preconditioning means for bringing the supply ofliquid coolant to a state of thermodynamic equilibrium at a temperaturethat is slightly lower than the predetermined load temperature, conduitfor conducting liquid coolant from the supply into heat-transferrelationship with the heat-emitting load 'to absorb heat therefrom,means for circulating the coolant through the conduit from the supply toan area of pressure that is lower than the pressure of the supply, meansfor sensing changes in the temperature-pressure conditions of thecoolant in the conduit, and controlling means that are responsive to thesensing means during heat transfer between the load and the coolant inthe conduit for adjusting the rate of coolant flow through the conduitto establish in the conduit a wet mixture of the coolant in a conditionof saturation at a temperature slightly below the predetermined loadtemperature. The preconditioning means may include a preconditioningchamber partially filled with liquid coolant, with the remainder of thechamber filled with vaporous coolant. The liquid and vapor in thechamber is maintained in thermodynamic equilibrium by automaticadjustment of the temperature and pressure within the chamber. Liquidcoolant in the chamber is circulated through the conduit to the loadthrough inlet and outlet metering valves at respective ends of theconduit. The temperature and pressure of the coolant within the conduitis adjusted so that all of the liquid coolant is boiling at all times.It is characteristic of the boiling process that the temperature of boththe liquid and vaporous coolant is the same and remains constant whenunder a constant pressure as long as there is any liquid coolantpresent. Thus upon the sensing means detecting any change in thetemperature-pressure conditions in the conduit, the rate of coolant flowis regulated accordingly by the controlling means, which includes theinlet and outlet metering valves, to ensure that a saturated wet mixtureof coolant exists throughout all sections of the conduit that is inheat-transfer relationship with the load. By keeping a saturated wetmixture in all parts of the conduit at all times, the temperature of thecoolant can be precisely controlled by maintaining the pressure constantsince the temperature of the boiling coolant will be uniformly at aconstant pressure. For even cooling of the load, it is preferable thatthe temperature of the coolant within the preconditioning chamber beonly slightly below that desired at the load so that the coolant willboil for the entire period that it is in heat-transfer relationship withthe load. To minimize heat leaks to the load, the inlet and outletmetering valves and other equipment are mounted within thepreconditioning chamber and thereby maintained in thermodynamicequilibrium with the coolant in the chamber. Thus any heat developed inthe valves or other apparatus is immediately transferred to ,theenvironment of .the preconditioning chamber which is automaticallycontrolled to maintain its temperature slightly below that of the load,thereby substantially eliminating heat leaks from such equipment to theload and providing immediate control over temperature differentials,particularly those that tend to develop in the valves.

It is an object of the invention to economically, accurately anduniformly maintain a heat load at a predetermined temperature and torapidly regulate the temperature to be constant under changing loadconditions, according to the invention. 1

Another object is to regulate the temperature of a heat load bymaintaining a coolant in a saturated wet-mixture condition at all pointsof heat transfer between the load and the coolant.

Another object is to eliminate heat leaks and temperature differentialsin a temperature control system.

Another object is to maintain a temperature control system including aheat load, coolant for the load, valves, and other apparatus in a stateof thermodynamic equilibrium.

Other objects and advantageous features of the invention will beapparent in a description of a specific embodiment thereof, given by wayof example only, to enable one skilled in the art to readily practicethe invention, and described hereinafter with reference to theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE isa schematic diagram of acontrol system for maintaining a heat load precisely at a predeterminedtemperature.

DESCRIPTION OF AN EMBODIMENT Referring, to the drawing there is shown inthe FIGURE a schematic diagram for a temperature control system for aheat load 5 mounted within aninsulated vacuum enclosure 7. A coolantsupply 11 is maintained in a thermally insulated preconditioning chamber9 and is supplied under pressure to the load through conduit such astubing 10. Liquid coolant is supplied to the chamber 9 from asupplementary liquid coolant supply 12. The level of the liquid coolantin the chamber 9 is regulated to be at a predetermined level by means ofa liquid level control 14 which is mounted in the chamber forcontrolling a metering valve 15 in a supply line 17 between the supply12 and the chamber 9. In normal operation, liquid coolant is suppliedfrom the chamber 9 to the tubing 10 through a two-way valve 18 via aport a to a port b and hence through an inlet metering valve 20, bothmounted in the chamber 9. An outlet metering valve 21 for regulating theflow of coolant from the load and hence the pressure and temperature inline 10 also is mounted in the chamber and is connected between theoutlet of the tubing 10 and venting tubing 23 which connects to an areaof pressure that is lower than the pressure in the chamber 9. Thus, nopumps having moving parts which would introduce localized heating arerequired. The ullage space in the chamber 9 is filled with vaporouscoolant; and all heating element 24 and a cooling element 26, poweredrespectively by a heater power source 27 and a control coolant supply28, a control valve 30 connected between the ullage space and an area oflower pressure, a temperature sensing element 31, a controller 32, and atemperature regulator 33.

The regulator 33 may be made responsive to signals from the element 31to either raise the temperature of thecoolant l1 byactivation of thesource 27 and element 24 or to lower the temperature of the coolant byactivation of the supply 28 and element 26. Further temperature controlis provided by manipulation of the valve 30 under control of theregulator 33 to adjust the pressure in the ullage space. When liquidhydrogen is used as the coolant 11, it is preferable that the coolingelement 26 and controlcoolant supply 28 be omitted and that thetemperature of the coolant 11 be lowered only by opening the valve 30under control of the sensor 31, controller 32, and regulator 33. When acoolant such as liquid freon or ammonia is used as the coolant 11, thenit is preferable that the element 26 and supply 28 be included in thesystem.

ln-order to stabilize the temperature of the chamber under changingcoolant flow-rate conditions and to make the pressure adjustments of thevalve 30 rapid and effective, it is preferable that the chamber 9 bearranged so that the liquid coolant therein has a large free-liquidsurface area that is equal to at least one-fourth of the total areabounding the liquid coolant, and that the liquid level in the chamber beadjusted to provide an ullage space that is filled with vaporous coolantand is relatively larger than the volume'of the liquid coolant 1 1. Therelatively large mass of vaporous coolant constitutes a stable pressureballastto damp pressure fluctuations and hence temperature fluctuations,while the mass of liquid coolant has a high thermal capacity andconstitutes a thermal ballast for damping temperature and hence pressurefluctuations. The large free-liquid surface area provides a largeevaporation area for rapid and effective heat transfer and acts as apressure balance between the mass of vaporous coolant and mass of liquidcoolant within the chamber 9. The coolant flow through the tubing 10 isregulated to be at a rate that sustains the coolant in its saturatedwet-mixture state in all sections of the tubing 10 during heat transferfrom .the load to the coolant. This regulation may be accomplished bymeans of temperature sensors 35 and 36 that are located in the tubingand a temperature sensor 37 at the load. Any change in temperature ofthe coolant in the tubing 10 is immediately detected by the sensor 35and a corresponding signal is transmitted through a transducer 39 to acontroller 40. The controller 40 is set at the desired load temperaturefor actuating another controller 42 to increase the opening of the valve21 upon detection of a rise in temperature from the set temperature andfor reducing the opening of the valve 21 upon detection of a drop intemperature. However, as long as liquid coolant remains in all sectionof the tubing, the liquid will boil at the saturation temperature duringheat transfer from the load to the coolant, and since there will be norise from the saturation temperature, the setting of the .valve 21 willnot change. Upon complete boiling away of liquid coolant at any point inthe tubing 10 the temperature and pressure throughoutthe tubing tends torise, thereby causing the opening of the valve 21 to increase and permita greater flow of liquid coolant. The tubing 10 is thereby maintainedpartially filled with liquid coolant along its entire length.

In order to prevent the system from hunting about the set predeterminedload temperature, signals from the sensor 37 a the load are transmittedthrough a transducer 44 to a rate controller 46 which gives an outputsignal proportional to the rate of temperature change at the load. Thisrate signal is applied to the controller 42 which is responsive theretoto modulate the signal from the sensor 35 in direct proportion to therate of load temperature change. Thus, the rate of change of the loadtemperature is determined and superimposed on the signal that controlsthe opening of the valve 21. If the load temperature changes slowly, therate of change of the signal to valve 21 is low to slowly change itsopening or closing. If theload temperature changes rapidly, the rate ofchange of the signal to valve 21 is high to rapidly change the openingor closing of the valve. Since in general the temperature at a loadtends to lag the temperature in the tubing 10, this particular manner ofcombining the signals in the controller 42 prevents the system fromoscillating or hunting about the set temperature. I

In order to provide more precise control over the rate of coolant flowthrough the tubing 10 and therefore more precise control over thetemperature in the tubing, the sensor 36 is located to transmit signalsthat correspond to the temperature in the tubing through a transducer 48to a controller 50 which controls the opening of the metering valve 20.The controller 50 is set at the predetermined load temperature forincreasing the opening of the valve 20 upon a rise in temperature beingdetected in the tubing 10 by the sensor 36 and for reducing the valveopening upon detection of a temperature reduction. 7

For best operation of the system, the coolant 11 is held in the chamber9 in a continuously boiling state at a temperature that is slightly lessthan the load temperature; in practice a few tenths of a degreetemperature difierence has been found to be satisfactory for liquidhydrogen as the coolant. Thus, as coolant is passed into heat-transferrelationship with the load, it continues to boil to dissipate the loadheat and thereby maintain the set load temperature, and since thepressure in the tubing 10 is held constant by means of the valves 20 and21, the temperature of the coolant remains constant at the saturationtemperature.

Preferably, the valves 20 and 21 are adjusted so that there is a slightflow of liquid coolant beyond the valve 21 into the tubing 23. Thisensures thatthere is always a wet mixture in the tubing 10 to providethe temperature-pressure regulation unique to the system. By keeping theliquid coolant flow in the tubing 23 slight, nearly all of the liquidcoolant is boiled at the load and only a small amount is vented to thetubing 23 which in turn is shown vented to the atmosphere. The slightflow results in maximum economic utilization of the liquid coolant whichmay be a substantial cost when liquid hydrogen is used as a coolant. Toachieve the desirable amount of liquid coolant flow through the valve21, a rate meter 52 may be inserted in the line 23 and the valves 20 and21 manually adjusted to an acceptable liquid flow rate before beingplaced under control of the controllers 42 and 50.

An alternative to venting the coolant to atmosphere, particularly in thecase of a coolant having a relatively high cost such as hydrogen, is toreliquefy the coolant and return it to the supply 12. Such means are notshown but are well-known in the art.

Where a liquid coolant such as a liquid gas is to be used and it has atemperature that is substantially different with respect to the initialtemperature of the load, a heat exchanger 54 may be inserted in the line23 and a line 56 to the valve 18, whereby the valve 18 may be turned todirect flow of coolant 11 from a third port c to the port a so that thecoolant l1 flows through the heat exchanger 54 to the tubing 10. Thecoolant to the load is preheated by the coolant in the line 23 from theload, thus reducing the differential between the incoming coolanttemperature and the initial load temperature to thereby the occurrenceof high stresses in the apparatus. By mounting the exchanger 54in thechamber 9, heat leaks from the exchanger to the line 10 and the loadduring normal operation of the system are eliminated.

In an embodiment exemplifying the invention, a preconditioning chamberhaving a total volume of 550 gallons was filled with 150 gallons ofliquid from a liquid hydrogen supply having an average temperature ofabout 25 K. The temperature and pressure of the chamber was regulated tobe 26.8 K and 88.0 psi respectively. The load was a liquid hydrogenbubble chamber as described in the Proceedings of the 1966 InternationalConference on Instrumentation for High-Energy Physics, published by theU.S. Atomic Energy Commission. The load was controlled to be within 1/150 K of a nominal temperature of 26.8 K. The inlet and outlet meteringvalves 20 and 21 were air-operated process control valves manufacturedby Precision Products and Controls, Incorporated of Tulsa, Oklahoma. Thesensors 31, 35, 36 and 37 were standard hydrogen-filled vapor pressurethermometers exemplifying a plurality of such sensors throughout thesystem. The transmitters 39, 44 and 48 were differential pressuretransmitters manufactured by Barton Instrument Corporation of MontereyPark, California. The controller 46 was a proportional, reset, plus ratecontroller and the controllers 32, 40, 42 and 50 were proportional plusreset controllers all manufactured by Moore Products Co. of SpringHouse, Pennsylvania. The tubing was of stainless steel having an insidediameter of onehalf inch. The heater power source 27 was of the variabletransformer type operating on 1 17 VAC; and the valve 30 was anair-operated process control valve manufactured by Preci-- sion Productsand Controls Incorporated of Tulsa Oklahoma. The temperature regulator33 was a two-position, pressure-actuated switch, manufactured by CustomComponent Switches Incorporated of Chatsworth, California. The systemwas operated without a cooling element 26 and control coolant supply 28.

While an embodiment of the invention has been shown and described,further embodiments or combinations of those described herein will beapparent-to those skilled in the art without departing from the spiritof the invention.

We claim:

1. A temperature-control system for precisely maintaining a heat load ata predetermined temperature, comprising:

a heat-emitting mass that is arranged as a heat load within said system;

a supply of liquid coolant;

preconditioning means for bringing said supply of liquid coolant to astate of thermodynamic equilibrium at a temperature that is below saidpredetermined load temperature;

conduit having an inlet and an outlet for conducting coolant from saidsupply into heat-transfer relationship with said heat-emitting mass toabsorb heat therefrom;

means for circulating coolant from said supply through said conduit;

means for sensing changes in temperature-pressure condition of coolantin said conduit; and

controlling means responsive to said sensing means for adjusting therate of coolant flow through said conduit to establish throughout saidconduit a wet mixture of coolant in a condition of saturation at atemperature slightly below said predetermined load temperature,

a portion of said controlling means being in heat-transfer relationshipwith the coolant circulating within said system, and

said preconditioning means including a preconditioning chamber with saidportion being mounted within said chamber to be maintained inthermodynamic equilibrium with the coolant contained in saidpreconditioning means.

2. The system of claim 1, further including,

means for maintaining said chamber filled with liquid coolant andvaporous coolant in the ratio of at least one volume of vaporous coolantto one volume of liquid coolant; and

wherein said chamber is arranged to provide the liquid coolant with alarge free-liquid surface area that is at least one-fourth of the totalarea bounding the volume of liquid coolant. 3. The system of claim 1,wherein said portion mounted within said chamber includes,

an inlet metering valve connected to said inlet of said conduit forcontrolling the flow of coolant from said supply into said conduit; andan outlet metering valve connected to said outlet of said conduit forcontrolling the flow of coolant from said conduit to a low-pressurearea.

4. The system of claim 3, wherein said inlet and outlet metering valvesare adjusted to provide a slight flow of said coolant in a wet-mixturestate beyond said outlet metering valve.

5. The system of claim 1, wherein said preconditioning means includesmeans for adding heat to said supply of coolant upon the temperature ofsaid coolant supply falling below said temperature that is below saidpredetermined load temperature, and means for removing heat from saidcoolant upon the temperature of said coolant rising above saidtemperature that is below said predetermined load temperature.

6. The system of claim 1, wherein said preconditioning means includesmeans for maintaining said supply of liquid coolant in a continuouslyboiling state.

7. The system of claim 1, further including means for preheating coolantcirculated from said supply through said conduit during initial coolingof said mass, said preheating means being mounted within said chamber tobe maintained in thermodynamic equilibrium with the coolant in saidchamber during operation of said system.

1. A temperature-control system for precisely maintaining a heat load ata predetermined temperature, comprising: a heat-emitting mass that isarranged as a heat load within said system; a supply of liquid coolant;preconditioning means for bringing said supply of liquid coolant to astate of thermodynamic equilibrium at a temperature that is below saidpredetermined load temperature; conduit having an inlet and an outletfor conducting coolant from said supply into heat-transfer relationshipwith said heat-emitting mass to absorb heat therefrom; means forcirculating coolant from said supply through said conduit; means forsensing changes in temperature-pressure condition of coolant in saidconduit; and controlling means responsive to said sensing means foradjusting the rate of coolant flow through said conduit to establishthroughout said conduit a wet mixture of coolant in a condition ofsaturation at a temperature slightly below said predetermined loadtemperature, a portion of said controlling means being in heat-transferrelationship with the coolant circulating within said system, and saidpreconditioning means including a preconditioning chamber with saidportion being mounted within said chamber to be maintained inthermodynamic equilibrium with the coolant contained in saidpreconditioning means.
 2. The system of claim 1, further including,means for maintaining said chamber filled with liquid coolant andvaporous coolant in the ratio of at least one volume of vaporous coolantto one volume of liquid coolant; and wherein said chamber is arranged toprovide the liquid coolant with a large free-liquid surface area that isat least one-fourth of the total area bounding the volume of liquidcoolant.
 3. The system of claim 1, wherein said portion mounted withinsaid chamber includes, an inlet metering valve connected to said inletof said conduit for controlling the flow of coolant from said supplyinto said conduit; and an outlet metering valve connected to said outletof said conduit for controlling the flow of coolant from said conduit toa low-pressure area.
 4. The system of claim 3, wherein said inlet andoutlet metering valves are adjusted to provide a slight flow of saidcoolant in a wet-mixture state beyond said outlet metering valve.
 5. Thesystem of claim 1, wherein said preconditioning means includes means foradding heat to said supply of coolant upon the temperature of saidcoolant supply falling below said temperature that is below saidpredetermined load temperature, and means for removing heat from saidcoolant upon the temperature of said coolant rising above saidtemperature that is below said predetermined load temperature.
 6. Thesystem of claim 1, wherein said preconditioning means includes means formaintaining said supply of liquid coolant in a continuously boilingstate.
 7. The system of claim 1, further including means for preheatingcoolant Circulated from said supply through said conduit during initialcooling of said mass, said preheating means being mounted within saidchamber to be maintained in thermodynamic equilibrium with the coolantin said chamber during operation of said system.